Gastroesophageal reflux of infancy is a common disorder which can have serious consequences. Despite its frequent occurrence during the newborn period, we understand little about the developmental factors that predispose the infant to gastroesophageal reflux. This project will examine the age related changes in mechanical properties and neurohumoral modulation of motility in the upper gastrointestinal tract. It will define the effect of developmental changes in shape, size, and geometry on the ability of the upper gastrointestinal tract to develop forces and pressures. The effects of these changes will be assessed through: 1). In vitro examination of tissue to determine force length and stress strain relationships of the developing esophagus, stomach, and their sphincters. 2). Isolated muscle cell techniques to determine the effect of age on cell size and the responsiveness to proposed neurohumoral transmitters. 3). In vivo studies to evaluate the effect of age on: a) Maintenance and modulation of upper esophageal sphincter, lower esophageal sphincter tone and relaxation, and esophageal peristalsis. b) Development of pressure volume relationships in the stomach. This systematic approach to age related changes in upper gastrointestinal motility will lead to more rational therapy for the common clinical problem of gastroesophageal reflux during infancy.